This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. A pilot study to compare systemic concentrations of cobalt and chromium in patients that have previously undergone total hip arthroplasties with ceramic-on-metal and metal-on-metal bearing surfaces. Secondary goals include an assessment for osteolysis on radiographs, potential long-term analysis of implant survivability, and cancer rates in the two patient populations.